The present invention relates to data storage media and devices, and more particularly to data storage devices including magnetic bit patterned media.
In conventional magnetic data storage media, data bits are recorded using magnetic transitions on a magnetic recording layer that is composed of a random arrangement of single-domain particles. That is, the magnetic recording layer is typically a thin film of a magnetic material that includes a random arrangement of nanometer-scale grains, each of which behaves as an independent magnetic element. Each recorded bit is made up of many (50-100) of these random grains.
A stream of data bits is recorded as regions of opposite magnetization on the magnetic recording layer. As recorded on the medium, the stream of bits generally consists of equally spaced bit cells, with a digital ‘1’ being indicated by a boundary (called a magnetic transition) between regions of opposite magnetization within a bit cell, and a ‘0’ being indicated by a continuous region without such a boundary. The boundaries between regions of opposite magnetization occur along the boundaries between the grains. As the magnetic transitions follow the grain boundaries, the transitions are typically not made along straight lines.
Thus, due to the granular nature of the recording layer, the transitions may not be placed exactly where they are intended. Any deviations in grain boundaries represent medium noise, which limits the density of data that can be recorded on the medium.
If the grains are small enough, the magnetic transitions may be straight enough that it is easy to detect which bit cells contain a boundary and which do not. However, if the recording density is increased for a given grain size, the magnetic transitions become proportionally noisier, reducing the ability of the system to accurately recover the data.
One way to reduce the medium noise is to reduce the grain size. However, due to the superparamagnetic effect, the grain size has a lower physical limit. The superparamagnetic effect refers to the tendency of a grain's magnetization to reverse when the product of the grain volume and its anisotropy energy fall below a certain value. That is, as the grain volume decreases, the magnetization of the grain can become unstable.
An alternative to conventional magnetic recording approaches is to use a bit patterned media (BPM) technique. In bit patterned media, the bits do not contain as many grains as those in conventional media. Instead, bit patterned media comprise arrays of magnetic islands which are defined on a nonmagnetic disk surface during manufacturing. The magnetic islands can be magnetized to a desired polarity one at a time by a magnetic field generated by a write head passing over the islands. The magnetic islands (referred to herein as “dots”) are physically separated from each other by regions of non-magnetic material. These nonmagnetic regions are referred to herein as “gaps” or “spaces.” Thus, the magnetic field generated by a write head in response to a write current can only change the magnetization of the dots, while the gaps remain unmagnetized. The magnetic islands can be formed, for example, through lithography when the disk is manufactured.
Each island, or transition between islands, may represent one bit of information. For example, a positive polarity may represent a data ‘1’, while a negative polarity represents a data ‘0.’ Alternatively, a transition from an island having a first polarity to an adjacent island having a different polarity may represent a data ‘1’, while a transition from an island having a first polarity to an adjacent island having the same polarity may represent a data ‘0.’ The signal-to-noise ratio of a bit patterned medium is determined by variations in the spacing and sizing of islands, and can be improved considerably beyond that of conventional media recording schemes.
FIG. 1A is a simplified diagrammatic representation of a top view of a disk 34 having a surface 36 which has been formatted to be used in conjunction with a sectored servo system (also known as an embedded servo system). Data is stored on the disks 34 within a number of concentric tracks (or cylinders, in the case of a multi-disk stack) 40a-h on the disk surface 36. Each track 40a-h is divided into a plurality of sectors 42 separated by radially extending spokes 43. Each sector 42 is further divided into a servo sector 42a and a data sector 42b. The servo sectors of the disk 34 are used, among other things, to accurately position the read/write head so that data can be properly written onto and read from the disk 34. The data sectors 42b are where non-servo related data (i.e., host device data) is stored and retrieved. Although FIG. 1A only shows a relatively small number of tracks for ease of illustration, it should be appreciated that typically tens of thousands of tracks are included on the surface 36 of a disk 34.
The servo sectors 42a in each track 40 are radially aligned with servo sectors 42a in the other tracks, thereby forming servo wedges 45 which extend radially across the disk 34 (e.g., from the disk's inner diameter 44 to its outer diameter 46).
FIG. 1B is a view of a track 40 including sectors 42, each of which includes a servo sector 42a and a data sector 42b, from the frame of reference of a read/write head of the disk drive. The cross-track direction (i.e., moving from the inner diameter ID of a disk to the outer diameter OD, or vice-versa) is perpendicular to the track 40, while the down-track direction is parallel to the track 40.
FIG. 1C illustrates exemplary servo information 80 that may be stored in at least some of the servo sectors 42a within the radial sectors 42. The servo information 80 can include various servo control fields, such as a preamble field 82, a servo address mark (SAM) field 84, a track number field indicated by its least significant bits (LSBs) 86, a spoke number field 88, an entire track number field 90 which may be recorded in at least one of the servo spokes, and a servo burst field 92 of circumferentially staggered radially offset servo bursts (e.g., A, B, C, D servo bursts).
FIG. 1D illustrates a BPM configuration including a regular array of rows 13 of patterned magnetic islands (i.e. dots) 11 on a disk surface 15. In the data sector 42b of a disk track 42, a write head can be moved along a row 13 of islands 11 and switched or pulsed with electric current to cause the desired recording of data by magnetizing each island to a desired polarization (e.g. a positive or negative polarization).